1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly, to an improvement of the material composition of a film of an aluminum alloy used as an interconnection of the semiconductor device.
2. Description of the Prior Art
Conventionally, as materials most widely used as an interconnection of a semiconductor device, an aluminum-silicon alloy (referred to as Al-Si alloy) containing approximately 1.0 to 2.0% silicon by weight has been known.
FIG. 1 is a cross sectional view showing a schematic structure of the conventional semiconductor device using the Al-Si alloy as an interconnection material. In FIG. 1, the conventional semiconductor device comprises an impurity diffused layer 2 formed in a predetermined region of the surface of a semiconductor substrate 1 of silicon, an underlayer insulating film 3 formed for the purpose of protecting and stabilizing the surface of the semiconductor substrate 1, an interconnection 5 of the Al-Si alloy (referred to as Al-Si interconnection hereinafter) formed in a predetermined region on the underlayer insulating film 3 and electrically connected to the impurity diffused layer 2 through a contact hole 4, and a completed protective film 6 formed on the Al-Si interconnection 5 and the underlayer insulating film 3 for the purpose of protecting the surface of the semiconductor device. A contact hole 7 is formed in a predetermined region of the completed protective film 6. The region having the contact hole 7 formed therein becomes a bonding pad region for electrically connecting the Al-Si interconnection 5 to the exterior.
FIGS. 2A to 2D are cross sectional views showing steps of forming an interconnection in the conventional semiconductor device using the Al-Si alloy shown in FIG. 1 as an interconnection material. Referring now to FIGS. 2A to 2D, description is made on a method for manufacturing the conventional semiconductor device.
In FIG. 2A, an impurity diffused layer 2 serving as an active region is formed in a predetermined region of the surface of a semiconductor substrate 1 using photolithographic techniques, an ion implantation process or the like. An underlayer insulating film 3 comprising a PSG (Phospho-Silicate Glass) film or the like is then deposited on the exposed entire surface using a CVD process for the purpose of protecting and stabilizing the surface of the semiconductor substrate 1. A contact hole 4 is then formed in a predetermined region of the underlayer insulating film 3 using photolithographic and etching techniques in order to make electrical contact to the impurity diffused layer 2.
In FIG. 2B, after a film of an Al-Si alloy is deposited on the exposed entire surface using a spattering process, a vacuum evaporation process or the like, an Al-Si interconnection 5 having a desired shape is formed using photolithographic and etching techniques.
In FIG. 2C, in order to obtain good ohmic contact between the interconnection 5 and the impurity diffused layer 2, heat treatment is performed for several ten minutes at a temperature of 400.degree. to 500.degree. C. in an atmosphere of hydrogen or oxygen, to cause an eutectic reaction in the interface of the Al-Si interconnection in the contact hole 4 and the semiconductor substrate.
In FIG. 2D, an insulating film such as a silicon oxide film, a PSG film and a silicon nitride film is deposited on the exposed entire surface as the completed protective film 6 using a CVD process. Then, in order to electrically connect the semiconductor device (Al-Si interconnection) to the exterior, a contact hole 7 is formed in a predetermined region of the completed protective film 6 using photolithographic and etching techniques. The region having the contact hole 7 formed therein becomes a bonding pad region.
FIG. 3 is a schematic cross sectional view of a semiconductor device using a simple substance of aluminum as an interconnection material.
When the simple substance of aluminum (pure aluminum) is used as an interconnection material, there is a phenomenon that in a heat-treating process at a temperature of 400.degree. to 500.degree. C. for forming good ohmic contact between an interconnection 10 of pure aluminum and an impurity diffused layer 2, silicon in the impurity diffused layer 2 and aluminum included in the interconnection 10 of pure aluminum locally react with each other in a contact hole region 4, so that an alloy pit 11 occurs, as shown in FIG. 3. This is not a problem when the depth of the impurity diffused layer 2 is large. However, the finer the semiconductor device is made, the smaller the depth of the impurity diffused layer 2 becomes, i.e., 0.5 .mu.m or less. Thus, this alloy pit 11 penetrates through the impurity diffused layer 2, so that a punch-through region 12 in the impurity diffused layer 2 is formed, whereby the interconnection 10 and the semiconductor substrate 1 are short-circuited. This reaction between silicon and aluminum takes place by the mechanism that at the time of heat treatment at a temperature of 400.degree. to 500.degree. C., the silicon in the impurity diffused layer 2 dissolves in the interconnection 10 of pure aluminum and the aluminum enters the impurity diffused layer 2 by mutual diffusion.
Conventionally, an interconnection of an Al-Si alloy having silicon previously added thereto in excess of the limit of silicon solubility to aluminum in the vicinity of a temperature of heat treatment, as shown in FIG. 1, has been widely used as one of the measures to prevent occurrence of this alloy pit 11.
The solid solubility (the limit of solid solubility) of silicon to aluminum is 0.25% by weight at a temperature of 400.degree. C., 0.5% by weight at a temperature of 450.degree. C. and 0.8% by weight at a temperature of 500.degree. C. The major trend is that the content of silicon in the Al-Si alloy which has been put into practice is slightly higher than the above described solid solubility, i.e., approximately 1.0 to 2.0% by weight.
However, even if the Al-Si alloy is used as an interconnection material as described above, the integration density of the semiconductor device is increased. Thus, as the size of a device is made finer to enter a submicron region, two large problems occur in the conventional Al-Si interconnection. More specifically, occurrence of the alloy pit in the contact hole region 4 can be prevented in heat treatment at a temperature of 400.degree. to 500.degree. C. performed after forming the Al-Si interconnection 5, as shown in FIG. 2C. However, other two defective modes appear.
One of the defective modes is a silicon deposition phenomenon that silicon included in the Al-Si interconnection 5 is deposited in excess in the contact hole region 4 by solid phase epitaxial growth in which substrate silicon serves as a seed crystal at the time of heat treatment. This deposited silicon 8 is close to an intrinsic semiconductor and has a very high specific resistance value. Thus, if silicon is deposited in a part or all of the contact hole 4 at a submicron level, contact resistance between the interconnection 5 and the impurity diffused layer 2 becomes very high, so that electrical failures occur.
The other defective mode is a phenomenon that silicon included in the Al-Si interconnection 5 in excess is deposited in the interconnection at a submicron level, so that a mass referred to as a silicon nodule 9 is formed. This silicon nodule 9 is close to an intrinsic semiconductor and has a very high specific resistance value. In addition, since the silicon nodule grows to the size of approximately 1 .mu.m, a cross-sectional area of an effective interconnection is locally decreased. Thus, the current density in this portion is substantially increased, so that failures such as generation of heat by electromigration and disconnection are liable to occur.